The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at You Know, That Thing Where.
If you don't mind approaching from the other direction, Hero Killer and Defeating the Undefeatable.
Edited by KD
I'm pretty sure the OP is looking for Sacrificial Lion.
^ I don't think it's Sacrificial Lion necessarily, Sacrificial Lion is about the character that dies to change the tone of the story being a fairly important and established character in terms of plot role, it has nothing to do with how physically powerful they are. A powerful hero who dies could still be a background character in the narrative, so would not count, and a physically weak character can still be a Sacrificial Lion.
Edited by molokai198
An incredible powerful hero dies.
Example: Suicide Squad: Kill the Justice League, when Arkham Batman is killed.